1. Field of the Invention
The present invention relates to an antenna receiver which can obtain satisfactory reception by combining signals received by a plurality of antennas, such as a diversity receiver or an adaptive antenna receiver of a base station.
2. Description of the Related Art
FIG. 3 shows the configuration of a known antenna receiver. Signals received by a plurality of (in this case, three) antennas 41, 42, and 43, which are placed with a certain distance therebetween, are amplified by amplifiers 44, 45, and 46, and are then frequency-converted to intermediate-frequency signals by mixers 47, 48, and 49, respectively. A local oscillation signal output from a local oscillator 50 is input to each of the mixers 47, 48, and 49 through phase shifters 51, 52, and 53 respectively. Thus, the phase of the intermediate-frequency signal output from each of the mixers 47, 48, and 49 varies in accordance with the phase of the local oscillation signal set by each of the phase shifters 51, 52, and 53.
Each of the intermediate-frequency signals is input to an adder 57 through band-pass filters 54, 55, and 56, respectively. The adder 57 adds the intermediate-frequency signals. The added intermediate-frequency signal is then demodulated by a demodulating circuit 58.
Herein, if the phases of the intermediate-frequency signals output from the band-pass filters 54, 55, and 56 are the same as each other, the power of the intermediate-frequency signal output from the adder 57 is maximized, and thus a demodulation signal having a sufficient amplitude can be obtained from the demodulating circuit 58. In order to maximize the amplitude of the demodulation signal, the demodulating circuit 58 controls and sets the phase of each of the phase shifters 51, 52, and 53 so that the power of the intermediate-frequency signal is maximized.
When an antenna receiver having the above-described configuration is applied to a system having many channels, such as a mobile phone, the local oscillator 50 needs to oscillate with a frequency according to each channel. Therefore, each of the phase shifters 51, 52, and 53 is required to have a broadband characteristic so that the phase can be changed over a wide frequency band. However, this involves higher costs.
Also, since the frequency of a local oscillation signal is often high, frequency loss is caused by providing lines for inputting the local oscillation signals to the mixers 47, 48, and 49 and by inserting the phase shifters 51, 52, and 53. As a result, the carrier-to-noise (C/N) ratio of the modulation signal deteriorates.